Various types of head mounted displays are known. An exemplary head mounted display (throughout the following specification the initials "HMD" may be used to mean "head mounted display") includes optics or optical components such as lenses, mirrors or the like, to direct the image from an image source to an eye or to the respective eyes of a person viewing the image (viewer). The image source develops and/or provides an image intended to be viewed and may or may not be part of the HMD. Head mounted display systems are used in the field of virtual reality and also in aircraft, for example, as part of a heads-up display system, and in other fields, too. Exemplary use of such display systems include three dimensional (3-D) or stereoscopic display and viewing features. Also, such systems may be used for simulators, such as flight simulators for pilot training or the like.
Many prior head mounted display systems use light emitting sources to create an image, such as a cathode ray tube, light emitting diode, etc. Several disadvantages to such light sources and head mounted displays using them are relatively large size, weight, and cumbersome nature. For example, in some virtual reality display systems, counterbalancing weights and support systems are needed to hold or to help to hold the helmet containing the virtual reality image source and optics so that it does not severely weigh down the head, neck, shoulders, etc. of the user.
In some prior display systems a modulator modulates light from a source; the images created are a function of modulation. A liquid crystal cell or liquid crystal display device may be such a modulator. A disadvantage of such modulating systems is the reduction in light output due to light blocking and/or absorption occurring in the modulator. To overcome such reduction in brightness, sometimes the intensity of the light source is increased, which draws additional energy, creates heat, requires a larger light source, etc.
Another disadvantage to prior head mounted display systems is the complexity of the components and of the arrangement of the components to provide the desired display or image output. Complexity, size, and so forth usually increase the cost for such systems and reduce the robustness of the system.
It is desirable that a display, especially an HMD, have adequate eye relief. and comfort with which images provided by the HMD can be viewed. One aspect of comfort is the distance at which the image is viewed; a comfortable viewing distance is about twenty inches or more, for example, approximate reading distance. An aspect of eye relief is the distance between the eye and the last optical element (such as the output objective of the display) closest to the eye; often it is desirable that such distance be relatively large to provide adequate eye relief. If adequate eye relief is not provided and/or if the viewing distance at which the image is seen is less than about twenty inches, then the eye may be strained to view the image, which may be uncomfortable and usually is undesirable.
The definition of eye relief also is described in Information Display, Vol. 10, No. 7 & 8, July/August 1994, pages 12-16, the article by Robert E. Fisher entitled "Optics For Head-Mounted Displays."
It would be desirable to reduce the size, weight and complexity of a display system, especially a head mounted display system.
It would be desirable to provide a relatively uncomplicated, small and robust display system, especially for an HMD.
It also would be desirable to provide a high quality image, e.g., bright and of good contrast and, if used, color, for viewing using an HMD, and especially to derive the image using a small display device.
Further, it would be desirable to obtain a relatively wide field of view in an optical display system, especially a head mounted one, and efficiently to deliver light produced by the light source to the viewer. Efficient delivery of light reduces the brightness requirement of the light source, energy requirements and output heat, while providing good brightness, resolution and contrast of the viewed image.
Additionally, it would be desirable to provide adequate eye relief in a head mounted optical display system.
Many prior HMDs and other displays have had a limited head box, which is the size of the area or location at which the viewer's head must be placed so the eyes see a desired image. For example, if a display were mounted on the head, the eyes are at a location which is relatively fixed to the place where an image is seen, such as a screen or image source, e.g., a projector and/or projection lens, liquid crystal display, CRT display, etc. In some circumstances it would be desirable to enlarge the head box to allow increased movement and positioning of the head while still permitting viewing of the displayed image.
Accommodation is the term used to describe the ability of the human eye to adjust its focus to various distances. This ability is used by the brain as an added cue in depth perception. If this cue is at variance with other depth cues such as vergences (stereo parallax) the brain will be confused, leading to eye strain and an increased susceptibility to motion sickness. It is therefore, highly desirable in 3-D display systems for all of the cues to be in agreement with one another. There must also be agreement between the camera system and the display system. The vergence is controlled by the spacing and toe-in of the cameras and projectors. These can easily be set in agreement. The accommodation is controlled by the focus of the camera lens. This can be set to the focal distance of the objects of interest by one of the standard auto-focus methods. All that is then required for proper accommodation is that the focus of the projector be set the same as that of the camera.